Metal-like properties, such as electrical conductivity, were first discovered in molecularly doped polyacetylene in 1977 (see Shirakawa et al., J. Chem. Soc. Chem. Commun., 1977, p. 578). Since that discovery, the characteristics of several electroactive polymers have been studied extensively.
The electrochemical polymerization of a monomer suitable for preparing an electroactive polymer was initially achieved utilizing pyrrole to form polypyrrole. It was determined that polypyrrole had an electrical conductivity of 100 Scm.sup.-1 and could undergo reversible oxidation and reduction by applying an electrical potential from about 0.8 volts to about -0.6 volts with respect to a saturated calomel electrode. Accompanying the oxidation/reduction process was an associated color change from blue-black to pale yellow, respectively.
The formation of electroactive polymer films on conductive substrates by electrochemical techniques has been found to produce polymer coated electrodes suitable for a variety of purposes arising either from the reversible oxidation and reduction of the polymer films or from the high electrical conductivity of the polymer films. As examples, electroactive polymer coated conductive substrates can be used to prepare electrochromic devices, "smart" windows, optical switches for information processing and charge coupled devices, electromagnetic interference devices, semipermeable membranes, catalytic electrodes, gas sensors, photovoltaic components, solid batteries, diodes, fast response non-linear optical materials, and electrostatic dissipation devices.
A severe limitation on the use of electroactive polymer coated conductive substrates for the purposes listed hereinabove, however, is the fact that electrochemically deposited conductive polymers are easily removed from conductive substrates by contact with a solvent or mechanically by moderate abrasion. Thus, the limited durability of electroactive polymer coated conductive substrates precludes their widespread use. Furthermore, the electrochemically deposited films generally have a non-uniform topography, limited solubility in common solvents, and widely ranging electrical conductivities from about 10.sup.-2 Scm.sup.-1 to about 10.sup.2 Scm.sup.-1.
Scanning electron microscopy data as well as optical micrographs show the presence of filamentary structures in electroactive polymer films, such as polypyrrole, which are electrochemically deposited directly onto conductive substrates, such as fluorine-doped or indium-doped tin oxide coated glass. It is believed that these structures represent areas of high density polymer growth resulting from an insufficient number of equivalent initiation sites, or the presence of preferred sites, on the surfaces of the conductive substrates. Weak adhesion then results from the small number of equivalent polymer nucleation sites, the absence of chemical bonds and/or weak van der Waals interactions, and stress imposed upon the nucleation sites by the film growth between filaments. Thus, electrochemically synthesized electroactive polymer films deposited directly onto conductive substrates are easily disengaged therefrom, and exhibit a considerable degree of surface and compositional non-uniformity.
Miasik et al., "Electronically Conducting Polymer Gas Sensors," Conducting Polymers, D. Reidel Publishing Co., 1987, p. 189 discloses a method for depositing a film of polypyrrole directly onto a gold film by the electropolymerization of pyrrole from an aqueous solution, to produce an ambient temperature detection device for several industrial gases. The resistance of the polypyrrole film so produced increases in the presence of ammonia and decreases in the presence of hydrogen sulfide. The durability of such a device, however, is limited due in part to the poor adhesion between the polypyrrole film and the gold substrate.
In Rubinstein et al., "Morphology Control In Electrochemically Grown Conducting Polymer Films. 1. Precoating The Metal Substrate With An Organic Monolayer," J. Am. Chem. Soc., 1990, 112, p. 6135, a monolayer of p-aminothiophenol was deposited onto the surface of a gold substrate to improve the adhesion thereto of an electrochemically grown polyaniline electroactive polymer film. The article states that the adhesion-promoting monolayer significantly increases the density of the electrochemically grown polymer film, and results in a radiation absorption coefficient at 6,000 Angstroms about eight time higher than the average adsorption coefficient for the same film grown on "bare" gold. The substantial increase in the electroactive polymer film density is attributed to the adhesion-promoting monolayer which facilitates and regulates the bonding between the modified gold substrate surface and the growing phase of polyaniline. It is stated that the beneficial effect obtained concerning film morphology is apparently caused by a more uniform and efficient nucleation-and-growth process on the treated surface, resulting in a film with significantly improved space filling. Thus, it is recognized that an adhesion-promoting layer between an electroactive polymer film and a conductive substrate provides the dual benefit of greater durability and increased polymer density. The deposition of a monolayer of p-aminothiophenol, however, is difficult to achieve and accurately control.
U.S Pat. No. 4,719,152 to Ohta et al. discloses an adhesion-promoting metal or metal oxide layer which is deposited onto a conductive layer such as, for example, indium-tin oxide. Thereafter, a pre-formed layer of electroluminescent material is mechanically jointed to the adhesion-promoting layer, to form a multi-layered structure which resists delamination of the conductive layer away from the electroluminescent layer. The electroluminescent layer is not, however, an electroactive polymer, but a zinc sulfide- and manganese-containing cellulose-type resin. Furthermore, the electroluminescent layer is not electrochemically grown on the exposed surface of the adhesion-promoting layer but is mechanically affixed thereto.
U.S. Pat. No. 4,153,529 to Little et al. discloses the use of a surface-modified amorphous carbon layer on an indium-tin oxide coated glass substrate, to achieve the uniform alignment of molecules in a liquid crystal material spread over the amorphous carbon layer. The amorphous carbon is scrubbed with neutralized ions to produce parallel microscopically fine grooves on its surface, which grooves induce parallel alignment of the liquid crystal molecules. Liquid crystal material, however, is quite different from the electroactive polymers contemplated by the present invention. Moreover, the liquid crystal material is merely spread over the amorphous carbon layer as opposed to being electrochemically deposited thereon.
It would be desirable to prepare by a simple process an electroactive polymer coated conductive substrate, having improved adhesion between the film and substrate and therefore greater durability, and having improved film properties such as greater density and uniformity and therefore greater conductivity.